Uncharted 4: A Thief’s End – Who is Sam?

Fans of Naughty Dog's Uncharted series are eagerly awaiting the highly anticipated and potentially final journey in Nathan Drake's journey, Uncharted 4: A Thief's End. One major surprise at Sony's Playstation Experience event back in December was the introduction of the character 'Sam', so what do we know about him so far?